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I am about to install an OS that may create its own ESP when I already have one. I am wondering which one will be actually used; will the firmware, according to the specification, choose one of them (say the first valid ESP that it finds), or maybe it will read data from both partitions and display it in boot menu?

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    You should only have one ESP per drive. It is ok for multiple installs to share that ESP. ESP actually uses GUID to know which partition is the ESP. Tools like gparted using boot flag are really setting the very long GUID in background. In Linux you can see GUID with sudo efibootmgr -v and lsblk -o name,fstype,size,fsused,label,partlabel,mountpoint,partuuid GUID is partUUID
    – oldfred
    Apr 5, 2020 at 14:13
  • @oldfred actually I have found this thread on FreeBSD forums forums.freebsd.org/threads/cant-boot-on-uefi.68141 where a poster shows that they have 2 ESPs, and, quoting this article happyassassin.net/2014/01/25/… , "UEFI is a permissive spec, and if you follow the design logically, there’s really no problem with having just as many EFI system partitions as you want." But I don't quite get it what that would look like on my system. But I understand the we should have one ESP. And then what if I have multiple drives? which esp is use
    – Artyom Gevorgyan
    Apr 5, 2020 at 16:02
  • I have seen where a user wanted two separate installs of Windows, so removed boot flag from first ESP and created second. Then able to boot both directly from grub. And since UEFI really uses GUID, it should find that. But some UEFI implementations only work with one ESP per drive. You just may have to test to see if your system is permissive or vendor did not implement it to allow that.
    – oldfred
    Apr 5, 2020 at 16:29

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Without a special file BOOTX64.EFI in folder BOOT on the ESP (I assume only the first ESP on a device counts here), most motherboards will not automatically detect bootloaders on the ESP at all, which means moving the drive to another device or resetting the UEFI firmware would cause it not to boot, and after starting the computer once with the device removed, the UEFI boot entry would also be normally gone.

Only one bootloader can occupy this default boot entry. Windows creates a new BOOTX64.EFI in BOOT automatically during installation, most Linux installers don't. Run grub-install --removable to do it afterwards.

For normal UEFI boot entries, which are created during bootloader installation, having the bootloader on a secondary ESP, even if it's on the same drive, is not a problem. At least in my extended experience, which includes very early UEFI implementations.

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